Plant Functional Traits

A functional approach to analyzing long-term change in plant communities

How do plant traits affect their likelihood of occupying different habitats and how they respond to deer browsing, climate change, and habitat fragmentation?

Biomes around the world are experiencing species losses, biotic homogenization, and invasions of non-native species. We are documenting these processes by systematically resurveying forest plant communities in the upper Midwest first surveyed 50+ years ago. Our work confirms that all three processes are occurring in the upland forests of Wisconsin even in the absence of direct habitat destruction or conversion. Differences among sites in the rates of species loss, homogenization, and invasion implicate particular drivers of ecological change including succession, habitat fragmentation, and herbivory by white-tailed deer. Exotic earthworm invasions, climate change, and other factors are also affecting community dynamics. We propose to extend this work in two important ways.

First, we will re-survey lowland forests subject to higher rates of growth and ecological turnover than the upland forests sampled so far. Accentuated changes at these sites should allow us to test more precisely the separate and combined effects of soil, light, invasive shrubs, and landscape conditions on plant community composition and dynamics.

Second, we propose to investigate the mechanisms driving shifts in community composition by exploring how plant functional traits affect the distribution and abundance of species within and among sites experiencing contrasting ecological conditions.
This work involves compiling an extensive database on a broad set of plant traits from published sources, lab analyses, and measurements in the field. These data will allow us to evaluate how ecological conditions interact with species characteristics to affect the dynamic processes of colonization, persistence, and local extinction. We will also use these data to test theories of community composition and dynamics based on neutral models and interactions between functional traits and stochastic factors.

Northern Wisconsin Upland Forests

Resurveys of plant communities provide valuable information on changes in species composition over time and clues about how species respond to environmental change. We report results from resurveys of 62 upland forest stands in northern Wisconsin and the western Upper Peninsula of Michigan fort survey around 1950. We identify plant species that have significantly increased or decreased in frequency in 1m2 quadrats (winners and losers) and evaluate the traits that distinguish these groups. Twenty-one winner species increased across sites (by25-400%), while 21 loser species decreased (by21-95%). Winners include both common native species and five invading exotics. Many are grasses or sedges are abiotically pollinated or dispersed (e.g., Carex, which increased 286%). Losers are mostly rarer native forbs that rely on animals for pollination and/or dispersal. Losers appear sensitive to desiccation, anthropogenic disturbance, and/or herbivory by white-tailed deer (e.g., Streptopus roseus, which decreased 73%). Declines in lower species are heterogeneous and stochastic across sites where as winners are more uniform and deterministic. Increases in common widespread native species account for most of the community change we observe across sites. The fact that winners resist or tolerate deer herbivory while many losers are sensitive to deer suggests that deer may be a key driver of the shifts we observe in these forests.

Southern Floodplain

The southern lowland forests of Wisconsin, as defined by Curtis’ Plant Ecology Laboratory, refer to forested wetlands that include floodplains and hardwood swamps. Floodplain forests, or fluvial swamps, are associated with rivers and streams. They experience significant fluctuations in moisture levels, and they may only flood every 2-3 years or longer, usually in the spring. Hardwood swamps occupy old lake beds that formed in depressions left by receding glaciers, and they tend to be wet for most of the year due to impeded drainage.

Floodplain forests are notable for diverse landforms that have resulted from frequent flooding and channel migration events. Different assemblages of plant species can be observed associating with different landforms positioned along the cross-section of the floodplain. These landforms, along with the floodplain-upland transition zones, may often support a more diverse plant community than adjacent upland forest communities.

Young floodplain forests are typically occupied by black willow and cottonwood in the overstory. These short-lived species are likely replaced by silver maple and to a lesser-degree green ash and American elm. Swamp white oak is perhaps the most charismatic floodplain species, especially in forests with well-drained soils. It requires light to regenerate, and so responds well to gaps created from flooding or even occasional fires. On less frequently flooded back terraces, you will begin to see basswood, bitternut hickory, hackberry, red oak, white ash, and other more typical upland species. Many of the wetter-spectrum species mentioned above, along with black ash and slippery elm, are typical of the lacustrine swamps.

The groundlayer of these lowland forests consists of a diverse mixture of forbs, sedges, grasses, and vines. Woody shrubs are a less frequent component of these dynamic systems, but can become quite dominant (e.g., buckthorn and prickly ash) in areas such as high terraces that have experienced a reduction in the frequency and/or duration of flooding. Head-high nettles (Laportea canadensis and Urtica dioica) are quite common in areas that are not directly subject to frequent flooding. Some of the ‘jewels’ of these lowland forests include cardinal flower, fringed loosestrife, and green dragon.

Southern Wisconsin Upland Forests

Although the replacement of oak (Quercus spp.) forests by more shade-tolerant species across Eastern North America is widely appreciated, little is known about its impact on understory species. We re-surveyed the under-and over-story species composition of 94 undeveloped stands in southern Wisconsin in 2002-2004 to assess shifts in canopy and understory richness, composition and heterogeneity relative to 50 years ago. Tree composition has shifted away from oaks (Quercus spp.) towards more mesic and shade-tolerant species (primarily Acer spp.). Most sites (80%) experienced declines in understory native species richness with mean species density declining 25% at the 1 m² scale and 23.1% at the 20 m² scale. Rates of native understory species loss are much greater in late-successional stands with conspicuous declines in remnant savanna species and those with narrow leaves. Initial overstory composition predicts changes in the understory better than changes in the overstory. Despite declines in absolute abundance, woody species have increased 15% in abundance relative to herbaceous species in the understory. Exotic species that occurred in 13 stands in 1949-1950 representing 5.5% of the flora now occur in 76 stands and account for 8.4% of the species present. Gains in exotic species richness and abundance are not closely linked to declines in native species richness or community heterogeneity. Although canopy succession has clearly influenced shifts in forest understory composition and diversity, these results suggest that understory dynamics are becoming decoupled from overstory dynamics as landscape effects start to play an increasing role.

Other Plant Communities

While our lab deals primarily with forest communities, other researchers have studied 50 years of change in other Wisconsin communities originally studied by John Curtis and the Plant Ecology Laboratory.